1. Field of the Invention
The present invention relates to a piezoelectric actuator, a liquid transporting apparatus which transports a liquid, and a liquid-droplet jetting apparatus which jets liquid droplets.
2. Description of the Related Art
As an actuator, of an ink-jet head for jetting droplets of an ink (ink droplets), which applies a pressure to a liquid (ink) in an ink channel, there has been known a unimorph piezoelectric actuator. Such a piezoelectric actuator, of the ink-jet head, includes a vibration plate which covers pressure chambers forming a part of an ink channel; a piezoelectric layer which is arranged on a surface of the vibration plate to face the pressure chambers; and electrodes which generate an electric field in the piezoelectric layer in a direction of thickness of the piezoelectric layer (thickness direction). When the electric field acts on the piezoelectric layer in the thickness direction, the piezoelectric layer is deformed to thereby generate a bending deformation in the vibration plate at a portion of the vibration plate facing the pressure chambers. This changes volume inside the pressure chambers to apply the jetting pressure to the ink inside the pressure chambers.
Here, for applying a substantial pressure to the ink inside the pressure chambers, it is necessary to substantially increase the change in the volume of the pressure chambers. For this purpose, it is effective to substantially increase an amount of displacement (displacement amount) of each of the piezoelectric layer and the vibration plate at an area facing the pressure chambers. For realizing this, it has been known to lower (reduce) a bending stiffness of such an area by forming a thin wall portion in a part of the vibration plate and the piezoelectric layer. For example, a piezoelectric actuator, described in FIG. 6 of Japanese Patent Application Laid-open No. H11-309864, is provided for a pressure chamber having a parallelogram shape in a plan view. In this piezoelectric actuator, a recess elongated or extending along a long side of the pressure chamber is formed on the inner surface of the vibration plate at an area facing an end in a direction of width of the pressure chamber (width direction; direction orthogonal to the longitudinal direction of the pressure chamber). Consequently, the thickness of the vibration plate is thinned (reduced) locally at the area of the vibration plate in which the recess is formed, and thus the bending stiffness is reduced (lowered) at this area.
For applying even more substantial pressure to the ink inside the pressure chamber by increasing amount of change in the volume of the pressure chamber when the actuator is driven, it is also important to increase, as much as possible, a length in the longitudinal direction of the pressure chamber, of a portion of the vibration plate at which the displacement of the vibration plate becomes the maximum (a portion, of the vibration plate, exerting the maximum pressure to the ink in the pressure chamber). However, in an actuator 500 hitherto know and as shown in FIG. 25, areas (first constrained areas) 505, of each of a vibration plate 501 and a piezoelectric layer 502, are constrained to almost same extent from three surrounding walls surrounding the first constrained areas 505 respectively in three directions, because the first constrained areas 505 are located near ends in the longitudinal direction of a pressure chamber 503 respectively, and are each located at almost a same distance from a wall surface 504a defining the ends in the longitudinal direction of the pressure chamber 503 and from a wall surface 504b defining ends in the width direction of the pressure chamber 503. Therefore, the first constrained areas 505 of the vibration plate 501 and the piezoelectric actuator 502 are not deformed as easily as areas (second constrained areas) 506, of each of the vibration plate 501 and the piezoelectric layer 502, which overlap with the ends the width direction of the pressure chamber 503 and which are constrained from only one direction (one side). Therefore, even when the bending stiffness of the first constrained area 505 of the vibration plate 501 and the piezoelectric layer 502 is same as the bending stiffness of the second constrained area 506 of the vibration plate 501 and the piezoelectric layer 502, the displacement amount of the vibration plate 501 becomes small in the first constrained area 505.
Therefore, even when a recess is formed in a portion of the vibration plate as in the piezoelectric actuator disclosed in Japanese Patent Application Laid-open No. H11-309864, a force of constraint (constraining force) from the surrounding is not changed in the first constrained area, and thus the displacement of the vibration plate is not large at the first constrained area. Therefore, when an attempt is made to change the volume of the pressure chambers more substantially when the actuator is driven to thereby apply a substantial pressure to the ink inside the pressure chambers, it is necessary to increase drive voltage of the actuator. In such a case, the cost of an electric equipment system is consequently increased.